1. Field of the Invention
The present invention relates to an optical device, an imaging device, a control method for the optical device, and a program applied to realize an imaging optical system suitable for an apparatus such as a monitoring-use digital camera or video camera.
2. Description of the Related Art
Conventionally, to find the subject (imaging target) and obtain a detailed image, it is normal to use a method in which a very wide-angle lens or fish-eye lens is used to image a wide field of view and the detailed image is obtained after distinguishing the target subject in the field of view.
The following is an example of the type of method used to acquire an image of such a subject.    (1) Method to obtain enlarged image of target subject using an electronic zoom.    (2) Method to obtain image of target subject by constructing an optical system as a zoom lens and changing the magnification of the optical system (zoom lens) at the telephoto end.    (3) Method in which a plurality of imaging systems are used, one being a wide angle imaging system and the other being a zoom imaging system, and the acquired image of the target subject is switched between the systems.
In another proposed method, the subject light directed to the imaging element is switched by driving a reflective member provided in the optical system (see, for instance, Japanese Laid-Open Patent Publication (Kokai) No. H9-297350, Japanese Laid-Open Patent Publication (Kokai) No. 2003-9104, and Japanese Laid-Open Patent Publication (Kokai) No. 2006-81089).
In a further proposed method, a reflected image in a peripheral part of a convex reflective member is used to observe the entire surroundings of the subject, and with the central part of the reflective member having a transparent construction, another image is observed using a separate optical system (see, for instance, Japanese Laid-Open Patent Publication (Kokai) No. 2006-139234).
However, the above-described conventional image acquiring methods have the following problems.
In the method of (1), to obtain the enlarged image of the target subject by extracting a part of the image, it is necessary to use an imaging element having a high number of pixels to obtain a highly detailed image, and the performance requirements for the imaging optical system are more stringent. This causes costs to increase.
In the method of (2), when the target subject is a moving body and the photographer sees the subject during zooming, it is necessary zoom out to a wide-angle image in order to follow the subject. It is not therefore possible to speedily observe of the subject. Moreover, it is difficult to realize an optical system which includes both wide-angle and telephoto functions.
In the method of (3), since a plurality of optical system are used, a plurality of camera units must be used. Hence, the size and cost of the imaging device increase.
In the method of (3) described above, in order to obtain characteristics resembling the characteristics seen when a plurality of imaging optical systems are used by switching a part of an imaging optical system using a single imaging element, a mechanism is typically required to insert/withdraw the part of the imaging optical system within the system. Conventionally, mechanisms for switching a part of an imaging optical system have been widely used in dual-focus type silver halide compact cameras. However, there is a problem in that inclusion of the mechanism to switch the part of the optical system complicates the construction.
In a further method, light is directed onto the imaging medium by combining a plurality of subject images using a half-transmitting reflective member capable of both transmitting and reflecting light. However, with this method there is a problem in that images darken due to the reduction in transmissivity, and in that the image is built up from a plurality of subject images, making it difficult to observe the target image separately.
Further, the technologies disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H9-297350, Japanese Laid-Open Patent Publication (Kokai) No. 2003-9104, and Japanese Laid-Open Patent Publication (Kokai) No. 2006-81089 have a construction which makes it difficult to implement large changes in imaging angle. Further, large changes in the direction of the target subject that is being imaged tend to occur. As a result, it is difficult to detect the target subject and acquire detailed information in a short time period.
Moreover, the technology disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-139234 has the problem that it is difficult and costly to manufacture a large reflective member with a favorable profile irregularity. Moreover, in Japanese Laid-Open Patent Publication (Kokai) No. 2006-139234, since the data values for implementing the optical construction is not disclosed, it is unclear if the device can be realized.